1. Field of the Invention
The present invention concerns a method of substrate temperature control, and a method of assessing substrate temperature controllability in a substrate processing apparatus, and in particular it concerns a method of controlling the substrate temperature which can be used in a substrate processing apparatus in which a substrate is held on a substrate holder by means of an electrostatic force and a heat transfer gas for substrate cooling purposes is passed between the substrate and the electrostatic chucking stage, and a method of assessing the controllability of the substrate temperature.
2. Description of Related Art
A conventional method of substrate temperature control in a plasma processing apparatus is described below with reference to FIG. 4 and FIG. 5. Any plasma source can be used in this plasma processing apparatus, and it is not shown in the drawing. In the drawings, reference number 101 is the process chamber, and the construction of its upper part is not shown in the drawing. A substrate holder 102 is arranged in the bottom part of the process chamber 101, and a substrate 103 is arranged on the substrate holder 102. The substrate 103 is held by means of an electrostatic chucking stage 104. The substrate holder 102 comprises a bias electrode 105 and a circulator 106, which circulates a cooling medium which cools the electrostatic chucking stage 104. A substrate bias electrode radio frequency power source 107 and a direct current power source 108 are connected to the bias electrode 105.
A gap is formed between the substrate 103 and the electrostatic chucking stage 104. An inert gas, such as helium (He) gas for example, is supplied into this space by means of a pipe 109. He gas is present and functions as a heat transfer gas, which enhances the thermal transfer characteristics between the substrate 103 and the electrostatic chucking stage 104 and cools the substrate 103. Moreover, reference number 110 is a conventional helium pressure control apparatus and reference number 111 is an evacuation pump which exhausts the He gas. The pressure of the aforementioned He gas is controlled by means of the helium pressure control apparatus 110 and the evacuation pump 111. The helium pressure control apparatus 110 comprises a helium pressure controller 112, a pressure gauge 113, a mass flow controller 114, valves 115 and 116, and a bypass valve 117.
The substrate 103 which is held on the substrate holder 102 by an electrostatic force is subjected to an etch process with the plasma which is generated by the plasma source. During this process, a radio frequency (RF) is applied to the bias electrode 105 from the RF power source 107, and a self bias voltage is generated at the surface of the substrate 103. A direct current (DC) voltage is applied from the DC power source 108, and an electrostatic force is generated by the potential difference between the DC voltage and the self bias voltage, and this holds the substrate 103.
The method of controlling the He gas pressure is described below. Thus, He gas pressure control is achieved by means of the helium pressure controller 112. The helium pressure controller 112 sends a set flow-rate value via a signal line 118 to the mass flow controller 114 and recognizes the measured pressure which is sent from the pressure gauge 113 via a signal line 119. Thus, the helium pressure controller 112 sends open or close signals via a signal line 120 when the measured pressure is displaced from the set pressure value, the bypass valve 117 is opened or closed, and the He gas pressure is controlled.
This is described in more detail below with reference to FIG. 5. When the substrate 103 is not being etched, the valve 115 is closed, the bypass valve 117 is open and the valve 116 is closed. Moreover, the set He gas flow rate of the mass flow controller 114 is set to 0 sccm, and the set pressure value for the He gas is 0 Torr. The He gas pressure control which is carried out during the etch process of the substrate 103 starts after the substrate bias electrode RF power source 107 has been switched ON. At this time, the valve 115 is switched from closed to open, the bypass valve 117 is switched from open to closed, and the valve 116 is switched from closed to open. For pressure control, a set flow rate value signal for 20 sccm He gas is sent from the helium pressure controller 112 to the mass flow controller 114, and the He gas pressure is brought up to the set pressure value of 15 Torr.
With this pressure control, no He gas flows after the He gas measured pressure value has reached the set pressure value. A small amount, for example some 0.5 sccm, of He gas leaks into the space inside the process chamber 101 from between the substrate 103 and the electrostatic chucking stage 104. The measured He gas pressure falls below the set pressure value. He gas in an amount slightly greater than the amount which is leaked out, for example 0.6 sccm, is passed, and a fall in the measured He gas pressure is prevented. When the pressure exceeds the set pressure value, by 5 Torr for example, the bypass valve 117 is opened and He gas is exhausted with the evacuation pump 111 until the measured He gas pressure reaches the set pressure value of 15 Torr. The bypass valve 117 is closed again when the measured pressure reaches the set pressure value. Subsequently, the operation of the region indicated by 121 in FIG. 5 is repeated and the He gas pressure is controlled until the RF power source 107 is switched OFF. With this pressure control, the valve 116 is switched from open to closed and the bypass valve 117 is switched from closed to open at the same time as the RF power source 107 is switched OFF. Moreover, the set flow rate of the mass flow controller 114 is set to 0 sccm and the set pressure value is set to 0 Torr. The He gas between the substrate 103 and the electrostatic chucking stage 104 is exhausted for a fixed period of time with the evacuation pump 111, and then the valve 115 is switched from open to closed.
In the conventional method of He gas pressure control, the control of He gas pressure during the interval 121 shown in FIG. 5 is carried out simply by opening and closing the bypass valve 117. However, fine control of the He gas pressure between the substrate 103 and the electrostatic chucking stage 104 by simply opening and closing the bypass valve 117 is very difficult in practice. The variability in the change in the measured pressure with respect to the set pressure value is considerable. As a result, a variability arises in the substrate temperature from substrate to substrate when substrates 103 are continually being subjected to an etch process. Such a variability of the substrate temperature results in a variability between substrates in the selectivity to the mask and the selectivity to the underlying layer which are sensitive to changes in the substrate temperature. As a result, the reproducibility of the etch profile is poor.
In general plasma processing apparatus with which etching is carried out, by-products which are formed during the etching process become attached to the electrostatic chucking stage as many substrates are etched repeatedly, the state of chucking between the substrate and the electrostatic chucking stage becomes inadequate and so the cooling of the substrate becomes inadequate and the substrate temperature rises. If the substrate etch process is carried out at a high temperature, then a problem arises in that the reproducibility of the etch profile becomes poor. In terms of this problem, execution of the etch process at high temperatures can be avoided if the etch process which is being carried out continuously is stopped when the state of chucking between the substrate and the electrostatic chucking stage becomes poor. However, with the conventional plasma processing apparatus described above there is no mechanism for determining whether the state of chucking between the substrate and the electrostatic chucking stage is good or bad, and so it is impossible to avoid execution of the substrate etch process at high temperature.
The problems described above are problems which occur generally in substrate processing apparatus.
An aim of the invention is to provide a method of substrate temperature control for a substrate processing apparatus with which the control of the heat transfer gas such as helium gas is improved, and with which the controllability of the substrate temperature is improved.
Another aim of the invention is to provide a method of assessing the substrate temperature controllability in a substrate processing apparatus where a heat transfer gas is being used, wherein the state of the substrate temperature control is assessed by monitoring the state of the gap between the substrate and the surface of the electrostatic chucking stage on which the substrate is arranged.
According to a method of the present invention, the pressure of the heat transfer gas which is flowing in the gap between the substrate and the substrate mounting surface of the substrate holder is measured and the flow rate of the heat transfer gas is controlled in such way that the measured pressure of the heat transfer gas becomes equal to a preset pressure value. Control of the substrate temperature is achieved in accordance with the heat transfer characteristics of the heat transfer gas which is flowing in the gap between the substrate and the surface of the substrate mounting surface of the substrate holder.
To execute this method of substrate temperature control, a means of establishing the target pressure of heat transfer gas (a pressure setting part) and a means for measuring the actual pressure of the heat transfer gas which is being introduced into the abovementioned gap (pressure gauge) are established in the structure of the apparatus. The set pressure value and the measured pressure are compared and the flow rate of the heat transfer gas is controlled on the basis of the difference between these values in such a way that the difference becomes zero. The control is carried out in such a way that the measured pressure rapidly approaches the set pressure value, and rapid control is achieved without giving rise to variability in the control.
The abovementioned method of substrate temperature control according to this invention is preferably such that the pressure control valve which has been established in the heat transfer gas flow way controls the flow rate of the heat transfer gas in such a way that the measured pressure becomes equal to the set pressure value with the input of a signal for the set pressure value from the pressure setting part and the input of a signal for the measured pressure from the pressure gauge.
The abovementioned method of substrate temperature control is preferably such that the abovementioned substrate is held on an electrostatic chucking stage which is included in the substrate holder.
According to one embodiment of the present invention, the pressure of the heat transfer gas which is flowing in the gap between the substrate and the substrate mounting surface of the substrate holder is measured, the flow rate of the heat transfer gas is controlled in such a way that the measured pressure of the heat transfer gas becomes equal to a preset pressure value, and then the state of the gap between the substrate and the substrate mounting surface is assessed on the basis of a comparison of this flow rate of the heat transfer gas and a standard value.
According to the present invention, it is possible to obtain information concerning the actual flow rate of the heat transfer gas for controlling the transfer gas flow rate. In terms of the actual flow rate of the heat transfer gas, the amount of heat transfer gas which leaks from the gap between the substrate and the electrostatic chucking stage depends on the size of the gap. Moreover, the size of this gap is determined by the state in which the substrate is held on the substrate holder. The actual flow rate of the heat transfer gas which is detected is monitored. The state of the thermal transfer characteristics in the abovementioned gap, which is to say the state of substrate temperature controllability, can be assessed by comparing this with a standard flow rate of heat transfer gas.
The abovementioned method of assessing substrate temperature controllability of this invention preferably assesses the substrate temperature controllability by assessing the state of electrostatic force between the substrate and the electrostatic chucking stage.
With the method of controlling substrate temperature of this invention, the set pressure value of the heat transfer gas and the actual measured pressure are compared and the heat transfer gas flow rate is controlled in such a way that the measured pressure rapidly becomes equal to the set pressure value, and so control of the heat transfer gas pressure is improved. Hence, the thermal transfer characteristics of the heat transfer gas can be maintained at the optimum level and substrate temperature controllability is improved.
With the method of assessing substrate temperature controllability of this invention, the flow rate of the heat transfer gas which is introduced into the gap between the substrate and the electrostatic chucking stage is monitored and, by comparing this with a standard flow rate, it is possible to assess whether the state of substrate temperature control using the heat transfer gas is good or bad.